One type of intraluminal medical device that is well-known in the medical profession is a self-expanding stent. Self-expanding stents are often used to treat blockages, occlusions, narrowing ailments and other related problems that restrict flow through body vessels. During delivery to a treatment site in a vessel, a self-expanding stent is typically constrained within a tubular delivery sheath. The sheath prevents the stent from prematurely expanding as it is directed through the vessel to the treatment site. Once in place at the treatment site, the sheath is retracted and the stent deploys automatically to an expanded configuration in which it exerts an outward radial force on the wall of the vessel.
Self-expanding stents are often fabricated from shape memory materials, such as equiatomic or near-equiatomic nickel-titanium alloys (e.g., Nitinol). A shape memory material may undergo a reversible phase transformation that allows it to “remember” and return to a previous shape or configuration. For example, a Nitinol stent may transform from a low-profile compressed configuration during delivery in a vessel to an expanded configuration at a treatment site by transforming from a lower temperature martensitic phase to a higher temperature austenitic phase. The phase transformation may be driven by a change in stress (superelastic effect) or temperature (shape memory effect). In practice, removal of the delivery sheath disposed about the Nitinol stent allows the phase transformation from martensite to austenite to occur at the treatment site. Accordingly, upon removal of the sheath, the Nitinol stent expands from a low-profile compressed configuration to an expanded configuration in support of the vessel. Nitinol may accommodate up to about 7% or 8% recoverable strain ε1, as indicated in FIG. 1A, which shows stress versus strain for a typical Ni—Ti shape memory alloy undergoing a stress-induced transformation between austenite and martensite.
To load a Nitinol stent into a delivery sheath, the stent may be cooled to a temperature at which it has a fully martensitic structure, and then it may be radially compressed to a low profile configuration. Typically, the stent is compressed in a compression apparatus and then removed from the apparatus for loading into the delivery sheath. If the stent is maintained at a temperature below an austenite start temperature (As) of the shape memory material as it is being removed from the compression apparatus, a phase transformation to austenite and expansion of the stent to its fully expanded configuration may be avoided. However, even if the temperature is sufficiently low to prevent a phase transformation to austenite, the stent may recoil (expand) a small amount ε2 when the compressive stress is removed, as indicated in FIG. 1B, which shows stress versus strain for a typical martensitic Ni—Ti shape memory alloy at a temperature below As. This recoiling may be sufficient to interfere with the loading process due to the small tolerance between the inner diameter of the delivery sheath and the outer diameter of the compressed stent, both of which are desirably kept as small as possible to minimize the profile of the delivery system. The recoiling stent may thus exert significant radial forces on the inner wall of the sheath during the loading process. Consequently, the stent may buckle or collapse during the loading process instead of sliding smoothly into the sheath. The recoiling of the stent once the force is released and the associated frictional forces during loading may be particularly problematic in the case of longer-length stents.
In view of these problems, the stent loading process may be improved by slowing or preventing the recoiling of the stent upon removal of compressive forces from the stent.